In an electrophotographic process, an electric latent image is formed on an image carrying member, the latent image is developed with a toner, and the toner image is transferred to a transfer material, such as paper, and then fixed with such a measure as heating and pressurizing. The toners used include not only a conventional monochrome black toner but also toners of plural colors for forming a full color image.
The toner is used as a two-component developing agent used after mixing with carrier particles, and a one-component toner, which is used as a magnetic toner or a non-magnetic toner. The toner is generally produced by a kneading and pulverization method. In the kneading and pulverization method, a binder resin, a pigment, a releasing agent, such as wax, a charge controlling agent and the like are melt-kneaded to form a mixture, and after cooling, the mixture is finely pulverized and classified to produce target toner particles. Inorganic and/or organic fine particles are added to the surface of the toner particles produced by the kneading and pulverization method according to purposes, thereby providing a toner.
The toner particles produced by the kneading and pulverization method generally have an irregular shape and a heterogeneous composition on the surface thereof. The shape and the surface composition of the toner particles finely vary depending on the pulverizing property of the raw materials and the conditions upon pulverizing, and it is difficult to control the shape intentionally.
When a material having high pulverizing property is used, in particular, the material is pulverized further finely to change the shape thereof through various kinds of stress in a developing device. As a result, in a two-component developing agent, fine particles of the toner are fixed to the carrier surface to accelerate deterioration in charging property of the developing agent, and in a one-component toner, the particle size distribution of the toner is broadened, whereby fine particles of the toner are scattered, and the developing property is lowered due to the change in toner shape, which deteriorate the image quality.
When the toner contains a releasing agent, such as wax, the releasing agent may appear on the surface of the toner since pulverization is liable to occur at an interface between the binder resin and the releasing agent. A toner having a combination of a resin that is hard to be pulverized due to high elastic modulus and brittle wax, such as polyethylene, often suffers exposure of the polyethylene on the surface thereof. The toner is advantageous in releasing property upon fixing and cleaning property of an untransferred toner from a photoreceptor, but the polyethylene on the toner surface may be released from the toner by a mechanical force, such as a shearing force in a developing device, and may migrate to a developing roller, an image carrying member, a carrier and the like. Accordingly, the developing roller, the image carrying member, the carrier and the like are liable to be contaminated with wax, which may bring about poor reliability of a developing agent.
Under the circumstances, in recent years, JP-A-63-282752 and JP-A-6-250439 propose an emulsion polymerization and aggregation method as a production method of a toner, in which the shape and the surface composition of the toner are intentionally controlled.
In the emulsion polymerization and aggregation method, a resin dispersion liquid is produced by emulsion polymerization, and separately, a colorant dispersion liquid is produced by dispersing a colorant in a solvent. The dispersion liquids are mixed to form aggregated particles having diameter corresponding to the toner diameter, which are fused by heating to provide toner particles. In the emulsion polymerization and aggregation method, the shape of the toner can be arbitrarily controlled from an irregular shape to a spherical shape by selecting the heating temperature conditions.
In the emulsion polymerization and aggregation method, toner particles are obtained by aggregating and fusing at least a dispersion liquid of resin fine particles and a dispersion liquid of a colorant under prescribed conditions. However, the resin that can be applied to the emulsion polymerization and aggregation method is limited in species, i.e., a styrene-acrylic copolymer can be favorably applied, but a polyester resin, which is known to have good fixing property, cannot be applied.
As a production method of a toner using a polyester resin, on the other hand, a phase inversion emulsification method is known, in which a pigment dispersion liquid and the like are added to a solution of a polyester resin dissolved in an organic solvent, to which water is then added, but the organic solvent is necessarily removed and recovered. JP-A-9-311502 proposes a method of producing fine particles by mechanical shearing in an aqueous solvent without the use of an organic solvent, but in this method, a resin and the like in a molten state are necessarily fed to an agitating device, which provides difficulty in handling. Furthermore, the method is low in degree of freedom on controlling the shape, and the shape of the toner cannot be arbitrarily controlled from an irregular shape to a spherical shape.
As disclosed, for example, in JP-A-2007-323071, when a mixture containing a binder resin and a colorant with a releasing agent added thereto is finely dispersed into particles in an aqueous medium, the releasing agent and the colorant are present on the surface of the toner, and thus the releasing agent and the colorant on the toner surface may be released from the toner by a mechanical force, such as a shearing force, in the developing device to migrate easily to the developing roller, the image carrying member, the carrier and the like although they are advantageous in releasing property upon fixing and cleaning property of an untransferred toner from the photoreceptor. Accordingly, the developing roller, the image carrying member, the carrier and the like are liable to be contaminated, which may bring about deterioration in image quality.